1. Field of the Invention
The present invention relates to a front light type touch panel-including illuminator which can be used for forming a touch-input type reflective liquid-crystal display device displaying an image hardly disordered, free from moire, bright and easy to view.
2. Description of the Related Art
As a related-art reflective liquid-crystal display device having a reflective liquid-crystal display panel, an illuminator constituted by a side light type light pipe, and an input position detecting touch panel provided on a visual side of the reflective liquid-crystal display panel through the illuminator, there is known a reflective liquid-crystal display device having a touch panel-including illuminator formed by integrating a touch panel with a side light type light pipe, the touch panel including a pressure-receiving side base material formed to have light-diffusing type light output means by mixing diffusing particles or printing a diffusing material (Unexamined Japanese Patent Publication No. 2000-162594).
Such a front light type reflective liquid-crystal display device having the illuminator disposed on the visual side is used in a mobile apparatus such as a cellular phone or a portable personal computer. According to the reflective liquid-crystal display device, emergent light of the light incident on a side surface of the light pipe is reversed in traveling course by a reflecting layer so that display light based on the reversed emergent light is viewed through the light pipe and the touch panel. Accordingly, a total reflective reflecting layer can be used and reflectance can be made high. Hence, the front light type reflective liquid-crystal display device has an advantage in that a display image can be made bright compared with a semi-transmissive liquid-crystal display device using a backlight unit and a half mirror.
In the reflective liquid-crystal display device, however, the touch panel and the side light type light pipe are integrally bonded to each other through a tacky layer, so that a gap between the touch panel and the side light type light pipe is filled with the tacky layer. For this reason, interfacial reflection in a surface of the light pipe or in a lower surface of the touch panel hardly occurs. Hence, lowering of display contrast is small but the light-diffusing type light output means hardly make light emerge in a direction perpendicular to the liquid-crystal display panel. Accordingly, there is basically a problem that effective light-utilizing efficiency is substantially considerably poor and luminance in a viewing direction is low.
There is a problem that the touch panel (particularly transparent electrodes of the touch panel) breaks down easily to shorten the lifetime of the touch panel. There is another problem that a display image obtained by the reflective liquid-crystal display device is disordered by light-diffusing dots to thereby result in reduction of resolution due to the image blurred. It is conceived that the former problem of easy breaking down is caused by the fact that the transparent electrodes are damaged and degraded easily by a concavo-convex structure of the light-diffusing type light pipe given to the pressure-receiving side base material when the transparent electrodes disposed to face each other through the input side and pressure-receiving side base materials are pressed to be brought into contact with each other so that position detection is performed by electrical current conduction between the transparent electrodes.
On the other hand, as the light output means, there are known portions formed into a prism structure of prism-like concavo-convex portions arranged like stripes at equal intervals, and portions constituted by convex or concave portions arranged like pits and each shaped like a cylinder in section. In the light output means in such a prism structure, bright display can be expected due to improvement of effective light-utilizing efficiency, but stress is concentrated on apices of the prisms to corrupt the prism structure at the time of inputting because the transparent electrodes also have prism-like concavo-convex portions. Hence, there is a disadvantage in that cracking or peeling of the transparent electrodes is apt to occur to thereby make the lifetime of the touch panel shorter.
In the front light system, the touch panel-including illuminator needs to have the following characteristics in terms of the visibility. That is, light can be made to emerge from the lower surface efficiently with good perpendicular directivity to thereby obtain excellent luminance; light leaked from the upper surface to disturb the visibility of a display image is weak; the display image is not disordered; external light can be transmitted efficiently; and the presence of the light pipe is not observed conspicuously. The light output means in the prism structure have a problem that the prism structure is apt to become conspicuous to disturb the visibility of the display image. The light output means in the prism structure have also a problem that the prism structure interferes with pixels of the liquid-crystal display panel to cause moire to thereby reduce display quality greatly.
As a countermeasure against moire, there has been proposed a method in which the direction of arrange of prisms is inclined to the direction of arrangement of pixels to thereby suppress moire. In the countermeasure, however, the moire-preventing effect is poor if the inclination angle is low. On the other hand, if the angle is high, light transmitted through the light pipe is reflected laterally by the prism surfaces. As a result, the countermeasure has a disadvantage in that the angle of emergence of light is inclined so largely that both the illumination efficiency and the light output efficiency are lowered.
On the other hand, in the light output means constituted by convex or concave portions arranged like pits and each shaped like a cylinder in section, light emerges at an angle largely inclined to the direction normal to the lower surface of the light pipe by light diffusion at the time of reflection in the same manner as the light-diffusing dots. Hence, the emergent light is poor in perpendicular directivity and not effective in illuminating the reflective liquid-crystal display device. That is, the reflecting surface of the reflective liquid-crystal display device generally takes a normal distribution type reflection form. Hence, reflectance increases as the direction of reflection approaches the direction of regular reflection. For this reason, if light emerging from the lower surface of the light pipe is poor in perpendicular directivity, light reflected by the reflecting surface can hardly illuminate the liquid-crystal display device effectively even in the case where emission efficiency on the lower surface is excellent. As a result, there is little contribution to improvement in luminance of the liquid-crystal display device, so that display is dark in the frontal direction. The direction of viewing the liquid-crystal display device made by the observer is generally equal to the direction normal to the liquid-crystal display device, that is, to the direction normal to the lower surface of the light pipe. Hence, light emerging in the direction normal to the lower surface of the light pipe illuminates the liquid-crystal display device most effectively.
In the light output means in the pit structure, portions having an inclination angle near to flat portions are formed continuously in the same manner as in semi-spherical dots. In reflection of external light, the light reflected by the portions having an inclination angle near to the flat portions reaches the observer's eyes continuously on the heels of the flat portions, so that the pits are apt to become conspicuous. There is a further disadvantage in that the presence of the light output means is apt to be recognized.